This invention is in the area of assaying uranium in formations traversed by a borehole. In particular, the invention relates to a method for improved uranium assay employing data from prompt fission neutron borehole logging, data from the measurement of formation density and data from other formation characteristics.
In the art of assaying earth formations for uranium deposits, several methods have been utilized. The most direct, time consuming, expensive and accurate method is to obtain a core of the earth formation and subject it to a laboratory analysis.
In order to decrease the time and expanse involved in obtaining a uranium assay, nuclear methods have been utilized in a borehole traversing the formation. One early method measures the natural gamma radiation in the borehole emitted by uranium decay products in the formation. This method is subject to many interferences and is not relied upon by industry for quantitative data.
Alternative logging techniques have been developed to obtain more reliable assay data. An analysis presented in a paper by Jan A. Czubek entitled "Pulsed Neutron Method for Uranium Well Logging," Geophysics, Volume 37, No. 1, February 1972, pages 160-173, disclosed that when a formation containing a uranium ore is irradiated with fast (14 MeV) neutrons, the formation moderates the fast neutrons to become thermal (less than 0.4 eV) neutrons. This thermal neutron flux induces fission in .sup.235 U, producing more neutrons with a fission spectrum that are designated as prompt fission neutrons (PFN). These neutrons may be measured when their energy is in the epithermal range. The amplitude of the epithermal neutron flux was stated to be proportional to the uranium concentration in the ore.
The Czubek model assumed an isotropic point source of neutrons, spherically symmetric geometry, a uniform matrix of uranium in the host formation, and a coincident source and detector. The expression for the response of a detect or of epithermal neutrons, R.sub..epsilon. is EQU R.sub..epsilon. =gkQ.sub.n h.sub.d t.sub.s .rho./(.SIGMA..sub.A).sup.2
where g is the amount of uranium (expressed as percent by weight), k is a collection of physical constants, Q.sub.n is a function of the neutron source output distribution, h.sub.d is a function of the neutron detector characteristics, .rho. is the formation density, .SIGMA..sub.A is the formation macroscopic thermal absorption cross section, and t.sub.s is the slowing-down time for neutrons of 14 MeV energy to epithermal energies. The Czubek relationship for the response of a thermal detector under the same assumptions is given by EQU R.sub..theta. =k'h'.sub.d Q.sub.n /(.SIGMA..sub.A).sup.2 + . . . ,
where the primed quantities are analogous to those defined above.
Initial attempts at log interpretation consisted of applying the Gzubek expression for R.sub..epsilon. to PFN data. Formation properties and probe operating properties such as detector characteristics, neutron generator output, etc. were estimated. These results were not satisfactory, with the grade implied by the calculated detector response being much less than the actual uranium grade, because the estimates of probe characteristics were not very accurate.
In an article entitled "Prediction of Time Dependent Neutron Fluxes Encountered In Pulsed Neutron Uranium Logging Experiments" in Nuclear Technology, Volume 31, page 133 (Oct. 1976), J. Renken suggested that a ratio of epithermal to thermal neutron fluxes should be less sensitive to trace neutron absorbers that affect direct PFN measurement. Calculations to substantiate this suggestion were published at page 119 of Transactions of the American Nuclear Society, 1976 International Meeting, November 14-19, 1976 by J. H. Renken entitled "Minimization of Neutron-Absorber Effects In Pulsed-Neutron Uranium Logging." One of the rationales behind this model can be seen from the fact that Czubek's thermal detector response would have many of the same uncertainties in probe characteristics which made application of the epithermal response formula unsuccessful. Consequently, the ratio of the two responses would cancel out many of these factors. This technique is callwed the "counts ratio" model.
The hardware utilized in the application of this model is described in detail in Sandia Laboratories energy report SAND77-0300, February 1977 (available from DOE Technical Information Center, P.O. Box 62, Oak Ridge, Tennessee 30830). Much of this hardware is also used in the present invention.
The counts ratio model was calibrated using the gamma ray calibration uranium test pits at the USDOE facility in Grand Junction, Colorado. However, on field data it produced inconsistent results. One of the reasons offered for the inconsistency of the counts ratio model was that the "thermal neutron" response being detected was actually gamma rays produced from the .sup.28 Si(n,.gamma.) thermal neutron capture reaction. The number of gammas so detected could vary with the content of silicon or other high-energy gamma ray emitters in the formation. These variations in the gamma ray production cause variations in the ratio that are not due to uranium concentration changes.
In U.S. Pat. No. 4,180,730 of Givens at al, a method of assaying for uranium in formations traversed by a borehole is disclosed and claimed in which the formation of interest is cylically irradiated by bursts of fast neutrons; the thermal and epithermal neutron fluxes measured during a time period when they are expected to be detected; and a ratio of the measurements of the thermal and epithermal neutron fluxes developed to give an indication of the concentration of uranium.
Givens employs analog measurement of the epithermal neutron counts and thermal neutron counts and uses analog techniques to obtain the ratio. The patent discloses the cyclic pulse rate of the neutron generator to be 1,000 pulses per second, leaving a spacing between each pulse of only one millisecond. Because this cyclic neutron pulse rate is so rapid, thermal neutron counts do not have sufficient time to return to background levels between pulses.
It has been discovered that knowledge of borehole size and earth formation factors is important for obtaining accurate uranium assay information using prompt fission neutron borehole logging. In particular, the point-by-point variations in the diameter of the borehole and point-by-point variations in earth formation factors, such as density and moisture content, are significant factors in obtaining accurate uranium assay information. Previously, these factors have been simply ignored or roughly averaged, thus compromising the accuracy of the results.
U.S. Pat. No. 4,209,694 to Mills disclosed a new logging procedure designwed to account for inaccuracies in Givens' patented ratio technique. This procedure involves logging a series of test holes having known concentrations of uranium ore and borehole diameter, taking the ratio of epithermal to thermal fluxes therein and using regression analysis to determine a correction factor that may be used to account for borehole effects from a field measurement.